"We're calling it a thump for two reasons: the high rate of snowfall and the type of snow that'll be coming down," NBC10 First Alert Weather meteorologist Steve Sosna said.

One to 2 inches of snow will be falling an hour. For some neighborhoods, the heavy snow will carry on for up to five hours, Sosna said.

The heavy snow will likely start in the late morning Wednesday in the Lehigh Valley and then spread south and east. The snowfall will abruptly slow to light flakes or stop completely. (Here's the latest expected snowfall totals.)

Sosna says the weather team has high certainty that large snow accumulations will pile up in the Lehigh Valley, Berks County and the upper suburbs in the Philadelphia ring counties. The level of certainty decreases around the Philadelphia area because the rain-snow line will hover near the Delaware River. That means totals could go up or down in and around Philadelphia.

The type of snow is also a contributing factor for this particular thump.

"Temperatures will hover right around freezing for the duration of the storm allowing the snowflakes to carry more moisture," Sosna said.

Heavy, wet snow weighs down trees and power lines. Don't be surprised to see additional power outages as trees weakened by the last nor'easter tumble.

This slushy, thick snow can also gum up train switches and paralyze rail lines. SEPTA has already warned riders that regional rail service will operate on a severe weather schedule, which means more limited service.